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Thermosensitive hydrogels based on PEG and poly(l-glutamate)s bearing different hydrophobic side groups were separately synthesized by the ring-opening polymerization (ROP) of l-glutamate N-carboxyanhydrides containing different alkyl protected groups, that is, methyl, ethyl, n-propyl, and n-butyl, using mPEG₄₅-NH₂ as macroinitiator. The resulting copolymers underwent sol–gel transitions in response to temperature change. Interestingly, the polypeptides containing methyl and ethyl showed significantly lower critical gelation temperatures (CGTs) than those bearing n-propyl and butyl side groups. Based on the analysis of ¹³C NMR spectra, DLS, circular dichroism spectra, and ATR-FTIR spectra, the sol–gel transition mechanism was attributed to the dehydration of poly(ethylene glycol) and the increase of β-sheet conformation content in the polypeptides. The in vivo gelation test indicated that the copolymer …